Nitric oxide decreases myofibrillar CA⁺⁺ sensitivity in skeletal muscle

Nitric oxide (NO) seems to modulate skeletal muscle function by inhibiting force production (Nature 373:546, 1994). In this study, we tested the hypothesis that NO would decrease force in isolated single skeletal muscle fibers by diminishing cytosolic Ca⁺⁺ concentration, [Ca⁺⁺]. Methods: Single skeletal muscle libers (n=17) were isolated from flexor brevis of male mice and microinjected with indo-1, a fluorescent Ca⁺⁺ indicator The responses of [Ca⁺⁺]_i and force to SNAC (250-1000 μM) were measured during brief (350 ms) submaximal contractions. Force/Ca⁺⁺ relationships were obtained by inducing contractions at different stimulation frequencies and the results fitted to a Hill equation. Maximal Ca⁺⁺-activated force was determined during incubation with 10 mM caffeine and 100 Hz stimulation, before and during exposure to SNAC. Results: Submaximal force showed large variability during exposure to SNAC, with a mean increase of 6% (P= not significant). [Ca⁺⁺]_i, on the other hand, increased from 466±21 nM to 524±37 nM (P<0.05). SNAC also changed the force/[Ca⁺⁺]_i relationship: the [Ca⁺⁺]_i giving 50% of maximal force (Ca₅₀) increased from 436±18 nM to 494±28 nM (P<0.05). Maximal force during SNAC exposure was 98.4±0.5% of baseline (P=not significant). Conclusions: We reject our original hypothesis because the NO donor SNAC increased [Ca⁺⁺]_i without affecting force. SNAC shifted the force/[Ca⁺⁺]_i relationship to the right, but it didn't alter the maximal Ca⁺⁺-activated force (during caffeine exposure). These results suggest that NO has two opposing effects on contractile function: it increases Ca⁺⁺ release and decreases myofibrillar Ca⁺⁺ sensitivity.